Hydrodynamic retarders have been used for many years as wear-free continuous brakes in motor vehicles on tracks as well as on the road, the latter in particular in lorries. Although such wear-free continuous brakes indisputably entail considerable benefits in terms of safety when braking the vehicle and in terms of reduced wear of the frictional service brakes, the no-load losses in non-braking mode of the hydrodynamic retarder constitute a critical point. Consequently, said no-load losses could still be decreased by providing so-called ventilation plates or by providing a rotor (primary wheel) moving out from the stator (secondary wheel) in non-braking mode, especially the latter measure is however insufficient normally to bring the no-load losses practically down to zero.
A possibility to bring down to zero the no-load losses of such a hydrodynamic retarder consists in designing the hydrodynamic retarder by means of a separating clutch to be disconnectable from the drive train. The following shortcomings can be thus provided; on the one hand, the time for closing the separating clutch is added to the time for filling the hydrodynamic retarder, which thanks to the hydrodynamic retarder lengthens the reaction time between the activation request for the hydrodynamic retarder and the supplying of the required braking torque. On the other hand, the separating clutch, which in particular is designed as a friction coupling, may due to the high constraints, in particular when switching on the hydrodynamic retarder, cause maintenance to be required early or components to be replaced compared to drive trains with hydrodynamic retarders, which are connected to the drive train without a separating clutch.
The European patent document EP 2 024 209 B1 suggest for shortening the reaction time of a hydrodynamic retarder connected to the drive train via a separating clutch, to preventively close the separating clutch every time there is no traction of the motor vehicle and to couple the retarder in emptied condition.
The disclosure DE 199 27 397 A1 suggests a self-reinforcing friction coupling for engaging the hydrodynamic retarder, which enables to engage the hydrodynamic retarder also in filled condition.
The disclosure DE 10 2005 052 121 A1 suggests switching off a hydrodynamic retarder by emptying its working chamber and simultaneously releasing the stator, so that the latter may spin with the rotor.
The disclosure DE 10 2009 001 146 A1 suggests a coaxial arrangement of the rotor of the retarder and of the rotor of an electric motor which can be disconnected together from the drive train via a separating clutch, in particular a desynchronised separating clutch.